In recent years, with the development of an information and communication technologies, various wireless communication technologies have been developed. Among them, a wireless LAN (WLAN) is a technology that can wirelessly access the Internet in a home or a company or a specific service providing region by using portable terminals such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), and the like based on a wireless frequency technology.
In order to overcome a limit for a communication speed noted as a weak point in the WLAN, IEEE 802.11n has been provided as a technological standard established comparatively recently. The IEEE 802.11n aims at increasing the speed and reliability of a network and extending an operating distance of a wireless network. In more detail, the IEEE 802.11n supports high throughput (HT) in which a data processing speed is equal to or higher than maximum 540 Mbps, and is based on a multiple inputs and multiple outputs (MIMO) technology using multiple antennas at both a transmitter and a receiver in order to minimize a transmission error and optimize a data speed.
In the STA, as the propagation of the WLAN is activated and further, an application using the WLAN is diversified, the necessity for a new WLAN system for higher throughput than a data processing speed supported by the IEEE 802.11n has come to the fore in recent years. A next-generation WLAN supporting a very high throughput (VHT) as a next version of the IEEE 802.11n WLAN system is one of IEEE 802.11 WLAN systems which have been newly proposed in recent years in order to support a data processing speed of 1 Gbps or higher in an MAC service access point (SAP).
The next-generation WLAN system supports transmission of a multi user multiple input multiple output (MU-MIMO) mode in which a plurality of non-AP STAs simultaneously access a channel in order to efficiently use the wireless channel. According to the MU-MIMO transmission mode, the AP may transmit a frame to one or more non-AP STAs which are MIMO-paired simultaneously.
Meanwhile, as the propagation of the WLAN is activated, an environment in which one AP provides a service to a lot of non-AP has been found. Moreover, as the number of mobile apparatuses that support the WLAN has increased, a method that can support increased mobility has been required.
As one example of an environment in which multiple STAs exist in one wireless LAN system, a machine to machine (M2M) network may be provided. The M2M network means a network that a machine supporting the WLAN communication sends and receives information as a principal agent, unlike the existing WLAN system in which there are a lot of cases in which humans receive services by accessing APs through STAs.
In the existing WLAN system, a flow of general communication is that an STA used by a user requests information through an AP and acquires information from the AP. In the general WLAN system, procedures for link establishment, such as scanning, authentication, and association are actively progressed through the request by the STA. The procedures are suitable for a communication flow in which the user requests information through the STA and acquires the requested information from the AP.
On the contrary, in the WLAN system that supports the M2M network, a general communication flow may be that the user requests information to a plurality of STAs associated with the AP through the AP and acquires information from each of the STAs. Under such an environment, the AP's being selectively associated with an STA that the AP will send and receive a wireless signal conforms to the aforementioned communication flow. However, when the procedures for link establishment start, it may be inappropriate for the AP to be selectively associated with the STAs. Therefore, a link establishment method which is appropriate to the M2M WLAN system is required.